Pericyclic Reaction

What is a pericylic reaction? Are there certain properties all possess? Is there more common ones I should memorize?

 

These questions and more will be answered shortly. Are you ready? Let's begin!

Concept: Properties and Types of Pericyclic Reactions

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Video Transcript

Hey everyone in this video I'm going to introduce you to an entirely new class of reactions that you haven't been exposed to yet called pericyclic reactions. So, guys we're just going to read this first sentence together and then I'm going to explain it, conjugated polyenes have the ability to react in non-ionic, concerted, cyclisation reactions that we refer to as pericyclic reactions, okay? Now, pericyclic reactions are an umbrella term for many different reactions but the crazy thing or the cool thing is that all those sub types of reactions all have the same properties in common. So, we're going to go through now is what those properties are that are shared by all pericyclic reactions, so the first one is non ionic, what does that mean? what that means is that many times with reactions in organic chemistry, we're used to having some kind of acid in some kind of base, some kind of electron donor an electron acceptor, right? But pericyclic reactions are unique because there are no ions, there are no partial charges, there's no negative and positive, nucleophile, electrophile it doesn't exist. So, solvents are going to have no effects on them since there are no partial charges, okay? Usually solvents might have actually influenced the rate of a reaction that has acid based qualities because there are charges that need to be stabilized but in this case there are no charges. So, solvents won't matter, they won't change the rate of your reaction, concerted, all bonds are created and destroyed simultaneously, there are no intermediate.

We've seen by now you might have seen other concerted reactions but this is going to be, these are all going to be concerted cyclization all these mechanisms to be a pericyclic reaction it needs to involve a ring of electrons around a closed loop forming a cyclic transition state. So, remember, keep this in mind, it has known intermediates but it does have transitions, transition states are what happened for concerted reactions and since these are cyclizations they should be cyclic transition states, reversible. So, all pericyclic reactions are reversible, so that means that we're always going to want to use our equilibrium arrows to describe them, this principle of reversibility is also referred to, in your textbook, as the principle of microscopic reversibility, that's just a complicated way to say that you can go in the right conditions you can go from reactants or products and from products to reactants and then finally all of these pericyclic reactions either occur thermally, meaning they're heat activated or photochemically, meaning that they are light activated, okay? So, just you're always going to have those two options to start off your reaction okay, cool? So, remember that I mentioned that there are several types of pericyclic reactions, what I'm going to do in this video is I'm going to introduce you to the three most common types, there are more than three types of pericyclics but the other like numbers four and five and six are for much more advanced organic chemistry courses and you're not going to be responsible for them, if you just know these three that might be, that's probably much more than sufficient, in fact, your professor or your textbook may not even teach you all these three. So, then in the next video I'll make sure only to include the ones that you need to know but for right now I think it's a good idea that you just know what the top three are and then we'll go ahead and the specific videos you see after this will be specific to the ones that you need to know, okay? So, pericyclic reactions can be easily categorized by the number of pi bonds that are destroyed after the cyclic mechanism. So, what I mean by destroyed is going from reactants to products okay, cool? So, let's talk about the first one, which is cycloadditions, okay? Cycloadditions are pericyclic reactions in which two pi bonds are destroyed after a cyclic mechanism. So, look at our reactants, right? How many pi bonds do we have in the first, in the set of reactants over on the left? we have three total, right? Two on the diene and then one on that alkene. So, what I would write here is I'd write Three reactant pi bonds.

Now, notice that I'm using an equilibrium arrow and heat to show that through a heat-activated cyclic mechanism, I'm going to form this product here, how many pi bonds does this product have? just one. So, how many pi bonds were destroyed? 2, this would be a cycloaddition any time that you're doing a cyclic heat or light activated mechanism to get rid of two pi bonds you automatically know it's a cycloaddition, cool? And there are, by the way, there are lots of different types of cyclo addition underneath that but in terms of the categories you could just say oh, this is a cycloaddition because two pi bonds are, not so bad, right? Let's go to the next type so the next category is electrocyclic reactions, electrocyclic reactions are pericyclic reaction in which one pi bond is destroyed after a cyclic mechanism, okay? So, in this case, this is a very typical electrocyclic reaction, I have three pi bonds to start off with then after I react that with heat in a reversal mechanism I would get only two pi bonds in my product. Notice that once again there was probably a cyclic type of mechanism because I'm forming a ring at the end and this means that this is an electrocyclic reaction because one pi bond is missing, cool? And then finally we have Sigmatropic shifts, Sigmatropic shifts are pericyclic reactions in which 0 pi bonds are destroyed after a cyclic mechanism. So, here. Notice that we start off with how many pi bonds? 2 and in the product, we have how many pi bonds? also 2. Now, the pi bonds did switch places and we actually did get a cyclic mechanism but all that happened was that the pi bonds change the position, it's not that the pi bond is gone completely. So, sigmatropic shift would be a heat or light activated mechanism in, which you're not losing any pi bonds and guys the cool thing is that just knowing these three little facts you would be able to distinguish, what cycloaddition, what's electrocyclic and what's sigmatropic, also something that you might have already noticed but I just want to draw to your attention so that you can memorize it easier, is that these happen to be in alphabetical order, which is nice. So, two bonds breaking is a cycloaddition then one bond breaking is an electrocyclic reaction and then 0 bonds would be a Sigmatropic shift, cool? So, they're in order C, E, S two, one, 0 and now you know in very general terms was that, what are different categories of pericyclic reactions, and once again all the properties that we talked about at the beginning applied to these three categories of reactions. Wonderful. So, we're done with this video, let's move on to the next one.

All pericyclic reactions share the following properties, regardless of the type:

Practice Questions:

Problem: Identify the type of following pericyclic reaction.

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Problem: Identify the type of the following pericyclic reaction.

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Problem: Identify the type of following pericyclic reaction.

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